The present invention relates to a method of preventing the formation of SO.sub.3 mists from oleum (fuming sulfuric acid).
Sulfuric acid and oleum are clear, colorless liquids which are also slightly turbid and oily in industrial grades. Whereas sulfuric acid is odorless, oleum develops white mists in the air. The physical properties of the various concentrations of the liquids can differ considerably. The commercial concentrations (sulfuric acid 78%, 96%; oleum 20%, 27%, 65%) are the result from the solidification point curve of the SO.sub.3 /H.sub.2 O system, which exhibits minima at each of these concentrations. The maxima correspond to defined compounds of SO.sub.3 with H.sub.2 O (e.g., H.sub.2 SO.sub.4.H.sub.2 O, H.sub.2 S.sub.2 O.sub.7).
Solutions of SO.sub.3 in sulfuric acid--oleum or fuming sulfuric acid have a considerable SO.sub.3 vapor pressure at normal temperatures already.
Sulfuric acid is a strong acid. Concentrated acid and especially oleum are oxidizing agents, especially at elevated temperatures. The acid is hygroscopic. Since a rather considerable amount of heat is released upon mixing with water (85.5 kj/mole H.sub.2 SO.sub.4 upon mixing with water at 20.degree. C.), the dilution reaction can be explosive. In addition, oleum forms dense mists with water. The water-removing action of oleum causes strong burn wounds on the skin of workers exposed to it and the skin heals poorly. In addition, wood and several organic substances are decomposed to carbon upon exposure to oleum.
Glass, enamel or ceramic materials (porcelain) are used to handle sulfuric acid or oleum of all concentrations, especially if contamination by foreign ions are to be avoided. When metals, plastics or rubber are used in the handling of these acids, precautions must be taken as to temperature and concentration of the acid. Lead protected by a layer of PbSO.sub.4 can be used as material for handling dilute acids up to approximately 85%. Other metals, even iron and steels, are attacked by weak acids.
Suitable materials for handling concentrated acids are grey iron, cast Si, steel, if a limited loss of metal is accepted in the bargain, or special steels (e.g., tool steel No. 1.4541 or 1.4571). Grey iron can not be used for handling oleum since the material is heavily corroded due to the oxidation of the carbon.
Rubber coated apparatuses or lines can be used for carrying acids up to approximately 70% concentration. However, PVC, polypropylene or polyethylene, which can even be used for cold sulfuric acid, are generally used today in this area. The fluorinated plastics PTFE or FEP are suitable for handling all concentrations of acid, even oleum.
Sulfuric acid and oleum cause severe skin burns and destroy tissue. The eyes of workers are especially at risk. Vapors or mists containing sulfuric acid and/or oleum result in severe irritations of the eyes, skin, respiratory system and lungs.
When working with sulfuric acid and oleum, protective equipment consisting of an acid-proof work suit and protective goggles must be worn for protection of workers. For work in which a direct contact with sulfuric acid or oleum is possible, additional protective equipment is to be used, e.g. face shield, rubber gloves, full protection on a PVC basis, heavy-duty respiratory protection system. If vapors or mists containing sulfuric acid or oleum occur, respiratory protective devices with P2 protective particle filters must be used, e.g. a full protective mask with ABEK P2 filter of the Drager company.
The value of the maximum workplace concentration (MAK value) (1984) is: EQU Sulfuric acid 1 mg/m.sup.3 =0.245 ppm
Odor threshold: EQU Sulfuric acid 0.6 mg/m.sup.3.
A number of different countries have adopted strict regulations when dealing with hazardous materials. For example, when working with oleum in Germany the notices of the German employer's liability insurance association apply: M004 notice "Irritating Substances/Burning Substances" (11-83 issue); M051 notice "Dangerous Chemical Substances" (4-84 issue); T015 notice about the emptying of acids and lyes from railroad tank cars (issue 71).
The dilution of sulfuric acid with water can be carried out only in such a manner that the acid runs slowly into the water, never the inverse, and attention must be paid to the dilution heat created in the process.
The open, exposed handling of all oleum types or the contact of oleum with moist air results in the formation of fine sulfuric-acid or oleum mists which must be avoided under all circumstances. An effective separation of such mists can only be achieved with fiber filters. The venting of containers or tanks should be secured via a sulfuric-acid washer with fiber filter.
Leaking or spilled acid and oleum must be trapped to the extent possible. Covering them with absorbent material such as dry earth or sand is possible. Sulfuric acid can be neutralized with soda, lime or ash, but oleum reacts in an extremely exothermal manner with these substances, generating a high level of evaporation of SO.sub.3. Only rather small amounts of acid can be washed away with water. Cloths, sawdust or other organic material can not be used to wipe up or absorb oleum or concentrated sulfuric acid.
On account of the strong tendency for the formation of mists due to the release of SO.sub.3 vapors into the environment, there is the necessity of neutralizing leaked oleum as rapidly as possible and of using a means which prevents the spread of SO.sub.3. It is known that leaked oleum can be covered with sand. However, sand has the disadvantage that it sinks immediately into the oleum, thus freeing the surface of the oleum for further evaporation. A further development of SO.sub.3 mist is then possible again. A further disadvantage is the fact that the oleum is not neutralized.
It is also known that oleum can be covered with a white oil (e.g., Winog 70). However, this method has the disadvantage that a disposal of the contaminated product thereafter is possible only by suction. Furthermore, this method can only be used conditionally with 30 to 40% oleum because at higher concentrations of the oleum the decomposition of the organic component occurs so rapidly that a sufficient covering is not achieved.
For the above reasons there exists in the art the problem of finding a method which effectively prevents the formation of SO.sub.3 mists and makes possible a problem-free disposal of the oleum.